The electroless deposition of a metal, e.g., a Group IB metal, i.e., copper, silver or gold, on either a metallic or non-metallic substrate usually requires pretreatment or sensitization of the substrate to render it catalytic to the reception of such deposit. Various methods have evolved over the years employing particular sensitizing compositions.
One of the most useful methods employs an aqueous solution consisting of two essential ingredients, a precious metal, e.g., palladium, gold, platinum, and the like, and a stoichiometric excess of a divalent Group IV metal, e.g., stannous tin. Such solutions are referred to as sensitizing solutions and often simply as seeders. Preferred sensitizing solutions are described in the said copending applications Ser. No. 53,352 (see also Canadian Patent No. 731,042); Ser. No. 285,889; Ser. No. 551,249; Ser. No. 712,575; U.S. Pat. No. 3,672,938, and Ser. No. 9,060, the disclosures of which are incorporated herein by reference. Especially useful forms of the sensitizing compositions, e.g., concentrates and dilutable solids, are disclosed in copending applications Ser. Nos. 9,060, and 50,918, the disclosures of which are also incorporated herein by reference.
A common problem with such sensitizing solutions, e.g., those of the palladium-stannous chloride type, has been instability. It appears that under the influence of air, oxidation of stannous tin to unusable stannic compounds or even insoluble stannic compounds tends to occur. Not only does this reduce the efficiency and effectiveness of the sensitizing solution, but it also seems to permit the precious metal to precipitate from solution, whereupon it forms a residue on any surface exposed to it. If the surface is a work-piece later to be electrolessly plated, such a flash coating reduces adhesion of the electroless metal. On the other hand, if the surface is part of the container holding the sensitizing solution, the precious metal will be lost from the bath and the process control is upset.
It has now been discovered that the addition of certain Lewis Bases, which are soluble in the sensitizer solution, will protect the sensitizer from decomposition.
While the reason for this useful effect is not clearly understood, it appears that the Lewis Base will react with and protect the reaction product of precious metal ion and Group IV metal ion to prevent reduction of the precious metal ion to free metal, e.g., precious in the colloidal state.
Merely by way of illustration, if hydroquinone, hydroxylamine, ethylene glycol, methanol and the like, are added to such sensitizing solutions, they dissolve and remain in solution, and any oxygen subsequently introduced into the solution from the air appears to be much less effective in causing the reaction product to be upset and decompose and, ultimately, precipitation of the precious metal is postponed or precluded.
It is unexpected to find that such Lewis Bases can be used as stabilizers without causing the sensitizing solution to become less useful for the desired purpose. It has been known, e.g., from Canadian Patent No. 731,042, that many precious metals are bound into a reaction product with the Group IV divalent metal, and it would be expected that extraneous compounds having an unshared electron pair (Lewis Bases) would tend to split or otherwise disrupt any such product. Lewis Bases as defined herein are compounds which provide a pair of electrons to form a new covalent bond by sharing them with an atom having an "open sextet" of electrons. Among the most common Lewis Bases are organic compounds containing oxygen, e.g., alcohols, ethers, phenols, hydroquinones, etc., nitrogen, e.g., ammonia, amines such as aniline, hydroxylamine, and the like, and many others.
Lewis Bases are defined herein in the same sense employed by standard works well known to those skilled in the art, e.g., G. W. Wheland, "Advanced Organic Chemistry", 2nd Edition, John Wiley & Sons, New York 1949, pp. 80-84, the disclosure of which is incorporated herein by reference.
Accordingly, it is a primary object of the present invention to provide stabilized sensitizer solutions that are highly active, as well as a process using such solutions to effect the sensitization of a substrate to render it catalytic to the reception of an electroless metal deposit.
Another object of the present invention is to provide new and useful stabilized compositions which are true solutions and methods for sensitizing substrates using them which substantially obviate the problem of non-adherent precious metal flash coatings.
It is an additional object of the invention to provide clear, stable sensitizing compositions and processes for the use thereof which materially reduce the time necessary to effect sensitization.
Additional objects and advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be realized by practice of the invention, the objects and advantages being realized and attained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.